Chemistry Q125 – AP-EAMCET 2025

Home Chemistry Chemical BondingQuestion #212

The increasing order of covalent character of NaCl(I), RbCl(II), MgCl

_{2}

(III), AlCl

_{3}

(IV) is

Clue 1 of 3: Fajan's Rules

The covalent character of an ionic compound is determined by the polarizing power of the cation and the polarizability of the anion.

🧠 Full Solution Path

Step 1: Identify the Cations and Anions✦ Active

The given compounds are NaCl(I), RbCl(II), MgCl $_{2}$ (III), and AlCl $_{3}$ (IV). All these compounds share the same anion, chloride ( $C l^{-}$ ). The cations involved are $N a^{+}$ , $R b^{+}$ , $M g^{2 +}$ , and $A l^{3 +}$ .

💡 Teacher's Secret Hint

Recognizing the common anion simplifies the comparison, as we only need to focus on the properties of the cations.

Step 2: Recall Fajan's Rules for Covalent Character○ Expand

According to Fajan's rules, the covalent character of an ionic compound increases with:

1. Higher charge on the cation (greater polarizing power).

2. Smaller size of the cation (greater polarizing power).

3. Larger size of the anion (greater polarizability). Since the anion ( $C l^{-}$ ) is the same for all compounds, this factor will not differentiate them.

💡 Teacher's Secret Hint

Fajan's rules help explain the deviation from ideal ionic bonding towards covalent character.

Step 3: Compare Cations based on Charge○ Expand

Let's compare the charges of the cations:

- $N a^{+}$ : +1

- $R b^{+}$ : +1

- $M g^{2 +}$ : +2

- $A l^{3 +}$ : +3

Since higher charge leads to greater covalent character, $A l C l_{3}$ (IV) will have the highest covalent character, followed by $M g C l_{2}$ (III). Thus, we have the partial order: $A l C l_{3}$ (IV) > $M g C l_{2}$ (III) > (NaCl (I), RbCl (II)).

💡 Teacher's Secret Hint

The charge of the cation is usually the most dominant factor in determining covalent character.

Step 4: Compare Cations with the Same Charge based on Size○ Expand

Now, let's compare $N a^{+}$ and $R b^{+}$ , both of which have a +1 charge. We need to consider their sizes.

- $N a^{+}$ is in Period 3.

- $R b^{+}$ is in Period 5.

Ionic size increases down a group. Therefore, $N a^{+}$ is smaller than $R b^{+}$ . According to Fajan's rules, a smaller cation size leads to greater covalent character. So, $N a C l$ (I) has higher covalent character than $R b C l$ (II).

Thus, $N a C l$ (I) > $R b C l$ (II).

💡 Teacher's Secret Hint

Remember the periodic trends for ionic radii: size increases down a group and decreases across a period for isoelectronic species.

Step 5: Determine the Overall Increasing Order○ Expand

Combining all the comparisons, the increasing order of covalent character is:

$R b C l$ (II) < $N a C l$ (I) < $M g C l_{2}$ (III) < $A l C l_{3}$ (IV).

This corresponds to the order: II, I, III, IV.

💡 Teacher's Secret Hint

Always list the compounds from lowest to highest covalent character when asked for 'increasing order'.

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Chemistry Question 125 – AP-EAMCET 2025

Clue 1 of 3: Fajan's Rules

🧠 Full Solution Path